This invention relates to a speed sensor, and more particularly to an electrostatic speed sensor for detecting, for example, the speed of a vehicle such as an automobile traveling on rotating wheels, the detection being based on the centrifugal force caused by the rotation of one of its wheels.
It has been known to electrostatically detect the speed of an automobile from the centrifugal force caused by the rotation of its tire. Japanese Patent Publication Tokkai 1996-240609, for example, disclosed such a speed sensor, characterized as having a weight attached to the back surface of a mobile electrode because changes in the acceleration as the running speed of the automobile is changed are not sufficient for displacing the mobile electrode of the sensor. A speed sensor of this type is not convenient, however, because the attachment of a weight onto the mobile electrode means an extra component to assemble and an extra work process in its manufacture and also because the finished product has a more complicated structure and the sensor cannot be made compact. In other words, such a prior art speed sensor cannot be easily attached to the tire of an automobile for detecting its speed by means of the centrifugal force thereon.
It is therefore an object of this invention to provide a structurally simple and compact speed sensor which comprises a reduced number of constituent parts and can be assembled with a reduced number of steps.
A speed sensor embodying this invention, with which the above and other objects can be accomplished, may be characterized as having a mobile electrode opposite a stationary electrode at a specified distance therebetween and displacing this mobile electrode by a centrifugal force to detect the speed. With a speed sensor thus characterized, there is no need to affix a weight, unlike the prior art technology described above, since the displacement of the mobile electrode is caused by a centrifugal force. As a result, the number of constituent parts and the number of steps required for the assembly process can be reduced, and the sensor as a whole is simpler in structure and can be made compact.
The mobile electrode may be supported by one or more hinge springs extending inward from a ring-shaped support structure. With the mobile electrode, the support structure and the hinge springs integrated, a compact speed sensor can be provided with a reduced number of components, and such a speed sensor can be easily assembled. Alternatively, the stationary electrode may be provided with a protrusion which penetrates an opening formed through the mobile electrode. With such a structure, even if a large external impulsive force is applied, the protrusion can support the mobile electrode and prevent any plastic deformation of the hinge springs. Such a protrusion may be formed to serve as a terminal for the stationary electrode so that the overall structure of the sensor can be simplified.
A spacer may be provided between the stationary and mobile electrodes such that they can face each other at a specified distance. This structure is advantageous in that the mechanical precision of the structure is improved, and sensors can be obtained with reduced variations in their operating characteristics.